Explanation will be made on the configuration of a conventional ultrasonic wave transmitting and receiving unit for use in an ultrasonic flow rate measuring apparatus.
FIG. 11 is a cross-sectional view showing the configuration of a conventional ultrasonic wave transmitting and receiving unit.
In an embodiment shown in FIG. 11, ultrasonic wave transmitting and receiving unit 100 is provided with top 60, side wall 61, case 63, piezoelectric member 64, vibration suppressor 65, holder 66, and vibration transmission suppressor 67.
Case 63 is formed into a topped cylindrical shape, and has side wall 61 and supporter 62 extending outward from side wall 61. Piezoelectric member 64 is fixed at the inner wall surface of top 60. Vibration suppressor 65 is fitted around side wall 61 in close contact, thereby suppressing vibrations occurring on side wall 61.
Holder 66 holds supporter 62. Vibration transmission suppressor 67 includes holder 66 therein, and fixes case 63 to channel 68 in a vibration suppressing manner.
Vibration suppressor 65 and holder 66 of vibration transmission suppressor 67 are formed integrally with each other (see Patent Literature 1).
FIG. 12 is a cross-sectional view showing a conventional ultrasonic wave transmitting and receiving unit having another configuration.
In ultrasonic wave transmitting and receiving unit 120, acoustic matching layer 71 having a thickness of ¼ of a wavelength λ of an ultrasonic wave is fixed at either surface of piezoelectric element 70.
Cylindrical case 72 constituted so as to surround piezoelectric element 70 is secured to acoustic matching layer 71, thus constituting ultrasonic sensor 73. Acoustic matching layer 71 is constituted by dispersing a glass balloon in a synthetic resin. Cylindrical case 72 is made of a plastic material having an acoustic impedance greater than that of acoustic matching layer 71. Moreover, cylindrical case 72 is filled with elastic resin 74 in such a manner as to embed piezoelectric element 70 therein (see Patent Literature 2).
However, with the configuration of ultrasonic wave transmitting and receiving unit 100, a radiation surface of piezoelectric member 64 is inclined if holder 66 is inclined in the case of being attached to an ultrasonic flow rate measuring apparatus, so that the direction of a propagation path of an ultrasonic wave may be shifted. In this case, there arises a problem to be solved that a propagation time, that is, measurement accuracy is adversely affected.
In contrast, like ultrasonic wave transmitting and receiving unit 120, a filler can secure reliability and facilitate the identification of a position of an ultrasonic wave radiating surface in the case where elastic resin 74 is filled in such a manner as to embed piezoelectric element 70 therein. However, there remains a problem to be solved with respect to a method for fixing unit 120 such configured as described above to a channel with little reverberant propagation.    PTL 1: Unexamined Japanese Patent Publication No. 2001-159551    PTL 2: Unexamined Japanese Patent Publication No. H10-224895